compiler: add function and global section pragmas

This patch adds a new pragma for functions and globals to set the
section name. This can be useful to place a function or global in a
special device specific section, for example:

  * Functions may be placed in RAM to make them run faster, or in flash
    (if RAM is the default) to not let them take up RAM.
  * DMA memory may only be placed in a special memory area.
  * Some RAM may be faster than other RAM, and some globals may be
    performance critical thus placing them in this special RAM area can
    help.
  * Some (large) global variables may need to be placed in external RAM,
    which can be done by placing them in a special section.

To use it, you have to place a function or global in a special section,
for example:

    //go:section .externalram
    var externalRAMBuffer [1024]byte

This can then be placed in a special section of the linker script, for
example something like this:

    .bss.extram (NOLOAD) : {
        *(.externalram)
    } > ERAM

compiler/compiler.go

@@ -762,6 +762,9 @@ func (c *compilerContext) createPackage(irbuilder llvm.Builder, pkg *ssa.Package
 			if !info.extern {
 				global.SetInitializer(llvm.ConstNull(global.Type().ElementType()))
 				global.SetVisibility(llvm.HiddenVisibility)
+				if info.section != "" {
+					global.SetSection(info.section)
+				}
 			}
 		}
 	}
@@ -787,6 +790,9 @@ func (b *builder) createFunction() {
 		b.llvmFn.SetVisibility(llvm.HiddenVisibility)
 		b.llvmFn.SetUnnamedAddr(true)
 	}
+	if b.info.section != "" {
+		b.llvmFn.SetSection(b.info.section)
+	}
 	if b.info.exported && strings.HasPrefix(b.Triple, "wasm") {
 		// Set the exported name. This is necessary for WebAssembly because
 		// otherwise the function is not exported.